Atoms are the smallest unit of matter that defines the chemical elements. Their size is measured in picometers – trillionths (10ˉ¹²) of a meter.

Protons and neutrons are called nucleons. Over 99.94% of the atom's mass is in the nucleus. The protons have a positive charge, the electrons have a negative charge, and the neutrons have no charge.

If the number of protons and electrons are equal, the atom is electrically neutral. If there are more protons or more electrons, then it has an overall positive or negative charge, and is called an ion.

The electrons don't move in the smooth orbits the image suggests. The areas where electrons might be found are called orbitals. Electron shells group electrons by energy level. Each shell can have one or more orbital.

A molecule is an electrically neutral group of atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge.
A molecule may consist of atoms of a single chemical element (eg oxygen - O2), or it may be a chemical compound composed of more than one element (eg water - H2O).

The world's smallest guitar is 10 micrometers long -- about the size of a single cell -- with six strings each about 50 nanometers, or 100 atoms, wide. Made by Cornell University researchers from crystalline silicon, it demonstrates a new technology for a new generation of electromechanical devices.

A chemical substance has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. Chemical substances can be chemical elements, chemical compounds, ions or alloys.

Chemical substances are often called 'pure' to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a lab. Other pure chemical substances are diamond (carbon), gold, table salt (sodium chloride) and refined sugar (sucrose).

Radioactivity is the process in which unstable atomic nuclei spontaneously decompose to form nuclei with a higher stability by the release of energetic sub atomic particles.

Alpha particles are commonly emitted by all of the larger radioactive nuclei such as uranium and thorium. Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Therefore alpha particles are positively charged.

Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei, such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (β). There are two forms of beta decay, β− and β+, which respectively give rise to the electron and the positron. The ilustration above represents an β− decay.

Gamma radiation, also known as gamma rays, and denoted by the Greek letter γ, refers to electromagnetic radiation of an extremely high frequency and consists of high-energy photons. Gamma rays are ionizing radiation, and are thus biologically hazardous. They are classically produced by the decay of atomic nuclei as they transition from a high energy state to a lower state known as gamma decay. Gamma rays have no electrical charge associated with them.

Both alpha and beta particles are deflected by an applied electrical field, while gamma rays are unaffected. Alpha particles are deflected less by the applied electrical field compared to beta particles, due to their higher mass.

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